Modulator driver circuit



p 1967 R. E WATSON 3,341,738

MODULATOR DRIVER CIRCUIT Filed Jan. 11. 1965 MODULATION EFFICIENCY DARK PERIOD PHOTOCONDUCTORS 51,41 m 3? PHOTOCONDUCTORS 39,43 L f OUT T INVENTOR S 2 ROBERT E. WATSON BY Q-C- WAN ATTORNEY United States Patent 3,341,738 MODULATOR DRIVER CIRCUIT Robert Elden Watson, Loveland, Colo., assignor to Hewlett-Packard Company, Palo Alto, Calif., a corporation of California Filed Jan. 11, 1965, Ser. No. 424,616 2 Claims. (Cl. 315201) This invention relates to a saturable core multivibrator circuit for driving gas discharge tubes which operate as the light sources of a photoconductor chopper modulator.

It is an object of the present invention to provide a circuit for alternately exciting the gas-tube light sources of a photoconductor chopper modulator with selected delay times between gas-tube excitations.

It is still another object of the present invention to provide an improved light source having low power consumption for use with a photoconductor chopper modulator.

In accordance with the illustrated embodiment of the present invention a saturable core multivi-brator receives a DC. signal which is regulated to maintain frequency stability and drives a pair of gas-discharge tubes alternately only on one-half cycle of the multivibrator output. This results in dark periods during the alternate half cycles when neither one of the tubes is lighted. The dark periods decrease total power consumption and increase modulation efiiciency by compensating for the time required for the photoconductors to change resistive values in response to modulated light excitation.

Other and incidental objects of the present invention will be apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIGURE 1 shows typical resistance changes of photoconductors used in a chopper modulator and also shows a graph of modulation efiiciency of a typical photoconductor chopper modulator as a function of the duration of the dark period of the exciting light sources; and

FIGURE 2 is a circuit diagram of the light source exciting circuit according to the present invention.

Referring now to the drawing, FIGURES 1(a) and (b) represent the resistance changes as a function of time of a pair of photoconductors conventionally connected to operate alternately and cyclically as a chopper modulator. It can be seen that such typical photoconductors change resistance expontially with time toward a final value from the time T when the light excitation (i.e. one on and one off) is reversed. This response time of the photoconductors has an increasing effect upon modulation efliciency with increasing modulation frequency, as shown by the curves for increasing frequencies to f.;. At some time t after the reversal of light excitation, the resistance of the two photoconductors is substantially equal. This tends to degrade modulator efficiency (i.e. the ratio of AC. peak modulator output to DC. modulator input). Introduction of a dark period (i.e. the portion of the excitation cycle during which no photoconductor is illuminated) between reversals of light excitations for the pair of photoconductors improves modulation efficiency, as shown in FIGURE 1(0), to a peak value at about 0.4 dark period. Thus operation at a dark period of 0.5 (i.e. the time during which no photoconductor is illuminated equals the time one photoconductor is illuminated) provides higher modulation efficiency at lower power consumption than operation with no dark time and can be readily accomplished using the circuit of FIG- URE 2.

FIGURE 2 shows a conventional saturable core multivibrator 9 which receives a regulated direct current from a low voltage supply 11 to produce a substantially symmetrical high voltage square wave at output terminals 13 and 15. A pair of series circuits, each including a neon glow discharge tube 17, 19 and a diode 21, 23, is connected across the output terminals 13, 15 to receive the high voltage square wave. The diodes 21, 23 are poled to conduct current only during the half cycle of square wave when terminal 13 is more positive than terminal 15. However, capacitor 25 connected between the common connections of a diode and glow tube in each series circuit insures that the glow tubes 17 and 19 ignite alternately on successive positive half cycles of the square wave. If, during operation, glow tube 19 is ignited, diode 23 conducts ignition-sustaining current causing a drop across limiting resistor 27. Capacitor 25 charges through diode 21 toward the positive output voltage (less the fixed voltage drop across ignited tube 19). When the output voltage goes negative diodes 21 and 23 are back biased and tube 19 is extinguished, leaving capacitor 25 charged more positively on the side connected to tube 17. This charge combines with the next positive output cycle to impress a greater net firing voltage upon tube 17 than upon tube 19 which insures that tube 17 and not tube 19 ignites. Capacitor 25 then charges in the opposite direction in a manner similar to that just described to insure that tube 19 ignites on the next positive output cycle.

The circuit including diodes 31 and 33 and capacitor 35 acts as a voltage doubler on the negative half cycle for applying a small additional positive voltage to the tubes 17 and 19 at turn on to insure that at least one glow tube ignites. However, the current regulator 10, which may be a passive device such as a varistor or an active transistor regulator of conventional design, causes the amplitude of the square wave output to increase greatly in the absence of a load (i.e. when no tube is ignited). This insures that the ignition voltage of one tube is reached and may obviate the need for the voltage doubler circuit just described. Glow tubes 17, 19 are shown optically coupled to photoconductors 3743 which are included in conventional photoconductor chopper modulator and demodulator of amplifier 45.

I claim:

1. A modulating light source comprising:

a generator for producing oscillations having alternate positive and negative half cycles;

a plurality of series circuits connected to receive said oscillations, each of said series circuits including a gas discharge tube and a diode poled for conducting current'during the same selected one of positive and negative half cycles of said oscillations;

a capacitor connected between a point intermediate a diode and tube in one of the series circuits and a point intermediate a diode and tube in another of said series circuits;

another series circuit connected to receive said oscillations and including a capacitor and a diode poled to conduct current during the other of said positive and negative half cycles of oscillations; and

a unidirectional conduction element connecting the common connection of said diode and capacitor in said other series circuit to said point in one of said plurality of series circuits.

2. A modulating light source as in claim 1 comprising:

a source of unidirectional voltage;

a saturable core multivibrator connected to said source for producing said oscillations having alternate positive and negative half cycles; and

a unidirectional current limiter connecting said multivibrator to said source for controlling the current supplied to said multivibrator.

(References on following page) 3 4 References Cited 3,191,039 6/1965 Gucwa 315-201 UNITED STATES PA NTS 3,235,748 2/1966 Mahoney et 211. 315-84.S

2,575,516 11/1951 Hagen 328-210 2,946,676 12/1960 Davies et a1 315-200 JOHN HUCKERT Examme" 3,040,195 6/1962 Jones et a1 30788.5 6 JERRY D, CRAIG, Assistant Examiner. 

1. A MODULATING LIGHT SOURCE COMPRISING: A GENERATOR FOR PRODUCING OSCILLATIONS HAVING ALTERNATE POSITIVE AND NEGATIVE HALF CYCLES; A PLURALITY OF SERIES CIRCUITS CONNECTED TO RECEIVE SAID OSCILLATIONS, EACH OF SAID SERIES CIRCUITS INCLUDING A GAS DISCHARGE TUBE AND A DIODE POLED FOR CONDUCTING CURRENT DURING THE SAME SELECTED ONE OF POSITIVE AND NEGATIVE HALF CYCLES OF SAID OSCILLATIONS; A CAPACITOR CONNECTED BETWEEN A POINT INTERMEDIATE A DIODE AND TUBE IN ONE OF THE SERIES CIRCUITS AND A POINT INTERMEDIATE A DIODE AND TUBE IN ANOTHER OF SAID SERIES CIRCUITS; ANOTHER SERIES CIRCUIT CONNECTED TO RECEIVE SAID OSCILLATIONSD AND INCLUDING A CAPACITOR AND A DIODE POLED TO CONDUCT CURRENT DURING THE OTHER OF SAID POSITIVE AND NEGATIVE HALF CYCLES OF OSCILLATIONS; AND A UNIDIRECTIONAL CONDUCTION ELEMENT CONNECTING THE COMMON CONNECTION OF SAID DIODE AND CAPACITOR IN SAID OTHER SERIES CIRCUIT TO SAID POINT IN ONE OF SAID PLURALITY OF SERIES CIRCUITS. 